A group-III nitride semiconductor, for example, a GaN (gallium nitride)-based semiconductor is expected as a material of a next-generation power semiconductor device. A GaN-based semiconductor device has a band gap wider than that of a Si (silicon) semiconductor and this may realize higher withstand voltage and lower loss than those of the Si semiconductor device.
In a GaN-based transistor, a HEMT (high electron mobility transistor) structure using two-dimensional electron gas (2DEG) as a carrier is provided in general. A normal HEMT is a normally-on transistor conducting even when a voltage is not applied to a gate. Therefore, there is a problem that it is difficult to realize a normally-off transistor conducting only when the voltage is applied to the gate.
In a power source circuit and the like handling a large amount of power of several hundreds to one thousand volts, normally-off operation is required with emphasis on safety. Therefore, a circuit configuration to realize the normally-off operation by cascode connection of a normally-on GaN-based transistor and a normally-off Si transistor is proposed.
However, in such circuit configuration, there is a problem of a break and deterioration in property of a device when an overvoltage is generated at a connection between the two transistors.